No, myregistry.foo.com/myimage is not possible. All images must be associated with a name space which is either a user or an organization. For e.g: myregistry.foo.com/admin/myimage or myregistry.foo.com/operations/myimage where operations is an organization created within DTR.

If you could not push an image under admin, it could be because you need to first create the repository and then try pushing the image into that repository. Or it could be that you have not logged in as a user who has permission to write into that repository.